Plant diseases cause considerable yield losses; this results in economical losses for farmers, but also a large amount of nutritional damage for the local population living off their agriculture. Economically and ecologically, it is very advantageous to have plants resistant to their pathogens, and more particularly to their fungi in the absence of plant-protection products. To date, it has been possible to use various strategies:                Methods of traditional selection have been used to develop plants specifically resistant to certain pathogens. However, these methods are limited to the species that can be crossed and the introgression of characteristics of resistance to pathogens constitutes long and laborious work.        The use of an antisense RNA makes it possible to decrease the expression of an endogenous target gene (EP 240 208).        The use of a sense gene makes it possible to decrease the expression of an endogenous target gene; this technology is called cosuppression (EP 465 572).        
The technology used in the context of the present invention is RNA interference or RNAi. RNAi has in particular proved that it is effective when double-stranded RNA (dsRNA) is injected into the nematode Caenorhabditis elegans (Fire et al. 1998, Nature 391: 806-811 and Montgomery et al., 1998, PNAS 95: 15502-15507, WO99/32619).
The expression in an organism of a sequence homologous to the gene of interest capable of inducing the formation of small double-stranded RNA makes it possible, very specifically, to extinguish this gene and to observe the phenotype that results therefrom (Xiao et al., 2003, Plant Mol Biol., 52(5): 957-66). The most striking example that illustrates this ability is that of insects fed with bacteria expressing small double-stranded RNAs corresponding to a gene expressed in the insects, which is thus inhibited (WO 01/37654).
The dsRNA triggers the specific degradation of a homologous RNA only in the region of identity with the dsRNA (Zamore et al., 2000, Cell, 101: 25-33, Tang et al., 2003 Gene Dev., 17(1): 49-63). The dsRNA is an RNA molecule which contains a double-stranded sequence of at least 25 base pairs (bp) including a sense strand and an antisense strand. The dsRNA molecules are also characterized by the very large degree of complementarity between the two complementary RNA strands. The dsRNA is degraded into RNA fragments of 19 to 25 nucleotides (siRNA) and the cleavage sites on the target RNA are evenly spaced apart by 19 to 25 nucleotides. The small siRNAs resulting therefrom exhibit a very high degree of identity with respect to the target RNA; however, mismatches of 3 to 4 nucleotides between the siRNA and the corresponding portion of the target RNA nevertheless make it possible for the system to operate (Tang et al., 2003, Genes Dev., 17:49-63). It has thus been suggested that these fragments of 19 to 25 nucleotides constitute RNA guides for recognition of the target (Zamore et al., 2000, Cell, 101:25-33). These small RNAs have also been detected in extracts prepared from Schneider 2 cells of Drosophila melanogaster which had been transfected with dsRNAs before cell lysis (Hammond et al., 2000, Nature 404: 293-296). The guiding role of the fragments of 19 to 25 nucleotides in the cleavage of the mRNAs is supported by the observation that these fragments of 19 to 25 nucleotides isolated from dsRNA are capable of being involved in the degradation of mRNA (Zamore et al., 2000, Cell, 101:25-33). Sizable homologous RNA molecules also accumulate in plant tissues which undergo the PTGS phenomenon (Post Transcriptional Gene Silencing, Hamilton and Baulcome, 1999, Science 286: 950-952). These small RNAs can regulate gene expression at three different levels:                transcription (TGS for Transcriptional Gene Silencing),        messenger RNA degradation (PTGS for Post Transcriptional Gene Silencing),        translation.        
Regulation involving messenger RNA degradation appears to exist in all eukaryotes, whereas regulation at the transcriptional level has only been described in plants, drosophile and C. elegans. As regards the regulation of translation, it has been characterized in C. elegans and drosophile and appears also to exist in mammals (Hannon, 2002, Nature, 418 (6894): 244-51). In the literature, reference is made to RNAi, to PTGS, to cosuppression or to quelling (reserved for fungi) when referring to this phenomenon, depending on the organisms in which it is studied.
The introduction of dsRNA was carried out in plants in order to induce silencing of an endogenous target gene (Hamilton et al., 1998, Plant J, 15: 737-746, WO99/15682), to induce resistance to RNA viruses by means of the use of a transgene expressing a dsRNA having substantial identity with respect to the viral genes (Waterhouse et al., 1998, PNAS 95: 13959-13964, Pandolfini et al., 2003, Biotechnol., 25; 3(1): 7, WO98/36083, WO99/15682, U.S. Pat. No. 5,175,102), but also to induce resistance to nematodes (Chuang and Meyerowitz, 2000, PNAS, 97: 4985-4990, WO01/96584) or alternatively to the bacterium Agrobacterium (WO00/26346, Escobar et al., 2001, Proc. Natl. Acad. Sci. USA., 98(23): 13437-13442).
In the case of the attack of a plant by a bacterium or by a virus, the mechanisms of interaction between the plant and the pathogen clearly involve nucleic acid transfers. In fact, in the case of Agrobacterium tumefaciens, the mechanisms of pathogenicity comprise two steps: the first corresponds to a horizontal gene transfer and to the integration of this or these gene(s) into the plant (this is transformation), the second corresponds to post-integration events that occur in the plant (this is tumorigenesis; Escobar et al., 2001, Proc. Natl. Acad. Sci. USA., 98(23): 13437-42) based on the use, by the plant, of the pathogen's genetic material. In the case of the infection of tobacco with the Plum Pox Virus (PPV), it is the transfer into the plant of the single-stranded RNA of the virus which allows the synthesis of the capsid proteins and of the polymerases required for the propagation of the infection (Pandolfini et al., 2003). There is therefore a link and very elaborate exchanges at the genetic level between the plant and its pathogen, and the siRNAs are transferred during its exchanges. The mechanisms of infection of a plant by a phytopathogenic fungus do not, for their part, involve any gene transfer.